Electrophoretic coating mixture



Patented May 15, 1945 ELECTROPHORETIC COATING MIXTURE Roscoe H. George and Wilfred M. Hesselberth,

West Lafayette, Ind., assignors to Radio Corporation of America, a corporation of Delaware No Drawing. Application March 16, 1942,

Serial No. 434,828

2 Claims. (01. 204181) This invention relates to electrode structures for use in electron tubes and is particularly directed to the manufacture and fabrication of the substantial extent the so-called keystoning effeet, which results when one of the scanning mosaic electrode used within electronic television scanning tubes. In its specific form; the invention is directed to a method and means for manu- ,facturing what is known as the image storage electrode element for a double mosaic electronic cathode ray tube for translating light images into electrical currents.

In the electron tube art, and particularly in connection with television transmissions, the socalled storage type pick-up tubes have found wide use for the purpose of converting light values into electrical representations, In such tubes there is provided an image storage element which is inthe nature of a multiplicity of minute size condensers. One electrode of such condensers may comprise a so-called signal plate which can be in the form of a continuous sheet, or the element may be in the form of awire mesh'or the like. The other electrode is usually in the form of minute size photo-sensitive elements which are separated .from the first-named electrode by some suitable form of insulator.

, In its operation, such a mosaic electrode, when subjected to light, releases electrons so that a charge is built up between the signal plate and the photosensitive elements with the insulator then serving as a dielectric. The arrangement is such that the produced electrostatic charge is proportional to the intensity 'of the impinging light. Whatever charge is built up on the mosaic electrode is released to an external circuit by the action of an electron beam or a cathode ray scanning beam, being directed so as to impinge upon the mosaic and caused to traverse the complete electrode element point by point in a suitable manner, such as is well known, so that the resultant electrostatic charge is released to an external circuit.

In some forms of'light translating tubes which are known as the single sided mosaic, the mosaic element is so arranged that the cathode ray scanning' beam and the light'eachimpinge from the same side. In other forms of the tube, the scanning beam and the light impinge from opposite sides, and such tubes are known as the double mosaic type. Some advantages are to be gained from causing the light and the scanning beam to impinge from opposite sides in that, in the centralmost point on the screen, both the light and the scanning cathode ray beam are adapted to impinge upon the electrode member along a path normal to the screen, This reduces to a beam or the impinging light rays strikes the mosaic electrode element along a path which is at an angle relative to the electrode member.

In the prior art it has been very diflicult to construct such double mosaic electrodes because of the fact that one electrode of the mosaic electrode element is usually formed from a mesh-like element, which is in the form of a fine mesh wire over which is placed a coating of some suitable form of insulator. The interstices of the mesh then are filled with some suitable form of conducting material, such as a silver paste or the like, as has been explained in the prior art, and after completion, the sensitization is then taken care of by coating the ends of the material filling the interstices of the mesh with a suitable photoelectrically responsive material.

In order to provide a scanning system wherein relatively high-definition pictures are obtainable, it is necessary to utilize a mesh-like base electrode element which is of the so-called fine mesh type, and this may be of the order of 400 strands of the mesh in each direction per inch, making 160,000 separate openings per square inch. Of course, it will be appreciated that each of these openings is extremely small and therefore, to coat each strand of the wire mesh with an insulating material becomes anexceedingly difilcult job.

It is accordingly an object of the present invention to provide ways and means bywhich the strands of the wire mesh used as a base member for such double mosaic electrodes may be uniformly coated.

A further object of the invention is to provide ways and means by which the coating for an insulating layer about each strand of the wire base member of a double mosaic structure, may be so placed upon the mesh that the interstices of the mesh, even though minute in size, shall remain unfilled by the deposited coating.

Other objects of the invention are to provide a double mosaic electrode structure in which an insulating layer of uniform thickness, and thus uniform capacity, is placed about a wire-like mesh, and in which the insulating material surrounding the wires of the mesh is caused to be deposited in the minimum amount of time.

Other objects and advantages will become apparent from considering the following specification in which the invention has been set forth in one of its preferred forms. In the disclosed embodiment, the invention comprises a system wherein the mesh-like element which is to be coated is immersed within a mixture from which the coating material is electrophoretically depos-- ited during the periods when an electric current is caused to flow through the mixture. In the preferred form,'the mixture is carried within a vitreous container and the mesh-like element to be coated lS placed in the mixture so as to be covered thereby, and it is then connected to one terminal of a suitable potential source. Also placed within the container is a second electrode member, preferably in the form or a copper plate, which is likewise connected to the potential source. The copper plate and the mesh are placed close together but at such a distance that when a voltage of the order of 7500 volts is applied between the electrodes there shall be no sparking.

In the preferred arrangement, the connections are so made that the voltage source shall be a direct current source with the positive terminal connected to the mesh electrode and the negative terminals connected to the copper plate. Within the vitreous container the coating mixture from which the coating for the mesh is to be obtained consists essentially of carbon tetra chloride (C014), barium fluoride (Bal 's), and a slight amount of castor oil. In starting the operation, the liquid contained within the vitreous container is preferably agitated so that all particle of barium fluoride are in suspension. It is noted that the coating material (barium fluoride) tends to settle out rather rapidly, but if the liquid has suficient density and viscosity the barium fluoride will be held in suspension for a length of time that is longer than the time required to coat the mesh. Since the viscosity of carbon tetrachloride is very low, the addition of a small amount of castor oil produces sufficient increase in viscosity in the mixture to enhance materially its ability to maintain the barium fluoride in suspension.

One mixture which has been found to be particularly satisfactory for the coating, is formed of a mixture of carbon tetrachloride (500 cc) and castor oil co) in which is suspended 4 to 5 cc. of finely powdered barium fluoride. For the voltage above specified of 7500 volts, it is found that the spacing between the mesh electrode and the copper plate may be of the order of about with the current permitted to flow through the mixture for a time period of the order of about 3 seconds. In order to provide the coating of barium fluoride upon the mesh element, the time during which the current is permitted to flow through the mixture is a function of the concentration of the mixture, the voltage applied and the spacing between the electrodes.

In the coating process, the particles of barium fluoride, when subjected to an electric field such as that applied between the mesh to be coated and the copper plate, tends to acquire-an electrostatic charge which. in the illustrated case, is negative. When the particles of barium fluoride have acquired these negative charges, they tend to be attracted by the positive potential applied to the mesh, and such particles as actually reach the mesh are held tightly against it in somewhat the same manner as if they had been. precipitated by Cottrell precipitation. The actual adhesion of the barium fluoride particles to the mesh is apparently substantially due to molecular attraction. Furthermore, in carrying out the coating process, it is believed that the particles of barium fluoride, when actually reaching the mesh, tend to increase the overall electrolytic resistance of the mesh at the areas where particles are deposited so that at uncoated areas (or areas with thinner coatings) the overall electrolytic resist--- ance is lowered and uncoated areas (or very thilll} coated areas) are subjected to greater current density until the desired uniform coating of barium fluoride is deposited on all sections or areas. The tendency found is for the particles of barium fluoride, which will subsequently be attracted to the mesh, to attach themselves to the mesh on the uncoated portions. In this way, the coating upon the mesh becomes more uniform. After the coating has once been electrophoreticaliy applied the electrode is removed from the mixture and permitted to dry. The mesh member which is to be coated is, for the example hereinabove suggested, usually formed from nickel, and under such circumstances, if the coating is permitted to dry, and then, after having dried, is baked for about one minute in a baking temperature of the order of 1300 degrees 0., it will be seen that the coated material is caused to melt, and then, upon cooling, to harden in a vitreous enamel upon the nickel wires of the mesh. The baking temperature hereinabove suggested is greater than the meltmg point for the barium fluoride, but less than that at which the nickel of the mesh will become fluid.

After the coated mesh has been subjected to heating and the barium fluoride particles have been melted at the high temperature they are caused to flow out over the metal of the mesh. In case there are any uncoated areas of the mesh after the first heating operation, it becomes immediately apparent, because of whathas been stated above, that the coated areas of the mesh, when again subjected to electrophoresis will so affect the coating that any further deposition of barium fluoride from the mixture tends to reach either the then uncoated sections or areas of the mesh or the coated sections on which the coating is thinnest, so that the thickness of the coating of barium fluoride applied to the mesh still further tends to become uniform.

For the purposes above suggested, it is de-= sirable to repeat the immersion, coating, drying and baking two to three times, with the face of the mesh-like element upon which the coating is to appear reversed in alternate coats relative to the copper electrode so that the deposit shall become especially uniform. After the last coating and drying and baking process, the electrode is then completed to the state where the remaining process of plugging the interstices and subse" quent sensitizing may be carried forward as disclosed, for instance, by the Flory or Hickok patents, No. 2,045,984 of June 30, 1936, and No. 2,047,- 36.9 of July 14, 1936, respectively.

In carrying out the process which has hereinabove been described, the system should be so constituted that the coating material which is to be applied to the mesh member is suspended in the insulating mixture of carbon tetrachloride and the oil which is of suitable density and viscosity in order that the particles of the barium fluoride may be held in suspension. There is some tendency for the suspended particles to settle out of the solution, but during the short time period when there is an actual application of the voltage which is to cause the particles to acquire a charge and to move toward the mesh which is to be coated, the particle of barium fluoride are held in suspension in the mixture in a manner not greatly unlike that of a colloid. In carrying out the process, it is desirable to keep the temperature of the solution at all times slightly above that'of the surrounding atmosphere in order to prevent moisture from collecting upon the particles which would influence the rate at which 2. The method of electrophoretically depositing a coating of insulating material upon a conductthe particles are able to attach themselves to the mesh.

While the proces has been described in one of its preferred forms, it is to be understood that many and various modifications may readily be made without departing from the spirit and scope of what is herein disclosed, and accordingly, the claims are to be considered in their broad and not in a limiting sense.

Having now described our. invention, what we claim is: I

l. A coating mixture for electrophoretically depositing an electrically insulating material, the

fluoride in the mixture.

castor oil added thereto, and particles of barium ing mesh, whichvcomprises the steps of positioning a.conduoting mesh member within a mixture consisting essentially of carbon tetrachloride with approximately 1% by volume of castor oil added thereto, and particles o'fbarium fluoride in suspension in the mixture, positioning a conducting member within the mixture at a predetermined spacing from the mesh member, applying an electrostatic field'between the mesh member and the conducting member with the polarity of the field being positive on the mesh member to be coated, and then continuin the application of the electrostatic field for a time period sufficient to attract suspended barium fiuoride particles to the surface of the mesh and to cause the said attracted particles to attach themselves to the mesh.

ROSCOE H. GEORGE; WILFRED M. HESSELBERTH. 

